1. Field of the Invention
This invention relates to fiber optic devices. More particularly, this invention relates to optical polarization beam combiner/spitters.
2. The Prior Art
Optical polarization beam splitter/combiners are known in the art. These devices may be used in optical communication in many ways including, but not limited to, optical power multipliers for combining two optical pump beams to increase optical pumping power, in coherent optical communications applications, and as optical polarization division multiplexers.
Two examples of prior art optical polarization beam splitter/combiners are shown in FIGS. 1A and 1B and FIGS. 2A and 2B, respectively. In the optical polarization beam splitter of FIG. 1A, an input beam enters from the fiber located at the left side of the figure. For the beam splitting operation, the incoming beam from fiber 1 at the left is collimated by the lens 1, and then enters the optical polarization beam splitter cube. The optical polarization beam splitter cube is able to split an arbitrary polarized light into two separated beams with orthogonally polarized directions. A first beam exits to the right and is coupled to fiber 2 through lens 2. A second beam exits in an upward direction and is coupled to fiber 3 through lens 3. In this way, an optical polarization beam splitter is realized. These three fibers could each be a single mode fiber, a polarization maintaining (PM) fiber, or even a multi-mode fiber.
The prior-art device depicted in FIG. 1A may be used as a bi-directional device if fiber 2 and fiber 3 are PM fibers, such that the optical beam polarization states coming from these two fibers are well defined and orthogonal to each other. Then these two beams can be added together at fiber 1 as shown in FIG. 1B. In this way, an optical polarization beam combiner is realized.
The device depicted in FIGS. 1A and 1B functions for its intended purpose. It however, suffers from several disadvantages, such as a larger device size necessitated by the need to employ an orthogonally disposed beam, suffer from a low extinction ratio, difficult to manage the fibers.
A second prior-art embodiment of an optical polarization beam combiner/splitter is depicted in FIGS. 2A and 2B. The optical principles of operation of the embodiment of FIGS. 2A and 2B are almost the same as those of FIGS. 1A and 1B.
Referring to FIG. 2A, a birefringent crystal is used to split the incoming arbitrary polarization light focussed from fiber 1 by lens 1 into two parallel beams having orthogonal polarization directions. These two beams are focused into fiber 2 and fiber 3, respectively, by lenses 2 and 3, such that the optical polarization beam splitter is realized. As shown in FIG. 2B, an optical polarization beam combiner can be realized as well. Compared to the embodiments of FIGS. 1A and 1B, the approach of FIGS. 2A and 2B provides high extinction ratio, employs fewer optical parts, and could be manufactured without the use of an optical epoxy in the optical path.
The device depicted in FIGS. 2A and 2B has its own drawbacks. Since fiber 2 and fiber 3 are located at the same side of the device, the birefringent crystal must have a length sufficient separate the two beams enough to accommodate the required spacing between lenses 2 and 3. Typically, lenses for this application have diameters of around 1.8 mm, requiring the minimum spacing between lens axes to also be about 1.8 mm. A birefringent must have a length of about 18 mm to provide the required beam separation of at least about 1.8 mm to accommodate the placement of lenses 2 and 3.